JP2574828B2 - Surface acoustic wave device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention suppresses unnecessary reflected waves generated at a characteristic impedance discontinuous change portion of a propagation medium, that is, at an electrode finger boundary portion, and reduces ripples in frequency characteristics. Further, the present invention relates to a surface acoustic wave device having asymmetric amplitude frequency characteristics.

[Conventional technology]

In the conventional cross-width-weighted interdigital transducer, unnecessary reflected waves (hereinafter abbreviated as MEL) due to the difference in characteristic impedance for surface acoustic wave propagation between the part where the electrode finger is present and the part where the electrode finger is not present are reported by the Institute of Electrical and Electronics Engineers of Japan. Magazine M
Tee Tee 22 (IEEE, Trans. MTT-22), 1974, No.
As discussed on pages 960 to 964, a method of suppressing the electrode finger width to 1/8 of the surface acoustic wave wavelength (λ ₀ ) at the center frequency has been conventionally known.

[Problems to be solved by the invention]

The above prior art does not consider the case of an unequal pitch electrode whose amplitude frequency characteristic is asymmetric with respect to the center frequency, that is, the pitch of the surface acoustic wave excitation source is not constant.
There was a problem that L was not sufficiently suppressed.

Even when the amplitude frequency characteristic is asymmetric and the electrode repetition cycle is not constant, the electrode finger width and space are set to 1/8 of the repetition cycle of the surface acoustic wave excitation source to achieve M
Although it is possible to suppress EL, this is insufficient when the asymmetry is large or when a high degree of suppression of the reflected wave is required.

An object of the present invention is to solve the above problems and to provide a surface acoustic wave device having a low ripple frequency characteristic by sufficiently suppressing MEL.

[Means for solving the problem]

In order to solve the above-mentioned problem, the present invention provides a method for generating a non-uniform pitch electrode having a nonuniform surface acoustic wave excitation source pitch.
An electrode finger having an electrode finger width equal to the electrode finger width of a pair of two electrode fingers having the same electrode finger repetition cycle having a constant surface acoustic wave excitation source pitch. By setting the same as the repetition period, a reflected wave in a predetermined frequency band generated at the electrode finger boundary where the characteristic impedance of the surface acoustic wave propagation medium changes discontinuously is suppressed.

[Action]

By making the repetition period of the electrode finger constant for both the equal pitch electrode and the unequal pitch electrode, at a specific frequency, two unnecessary reflected waves (MEL) due to the geometric shape of the electrode finger are generated. A sufficient MEL suppression degree can be obtained even in the vicinity of a specific frequency, offset by the electrode finger.

(Implementation)

FIG. 1 is a schematic plan view of an IDT according to a first embodiment of the present invention. The interdigital electrode 1 is used as an output electrode as an intersection width weighting electrode in which the intersection width W of the electrode fingers is not constant. The input electrode (not shown) has an opening length W of 1500 μm.
In m certain normal type electrode width of the electrode finger and a space is lambda _0/8
(Λ ₀ is the surface acoustic wave wavelength at the center frequency) of a split connect type. The center frequency is 56.5 MHz, and the logarithm of the surface acoustic wave excitation source is 15 pairs.

The weighting electrode of the present embodiment has a repetition period P _{1 of the} electrode finger.
And P ₂ is constant at 17 .mu.m, the ratio with respect to the electrode fingers repetition period此electrode finger width a (metallization ratio) is not constant. The number of SAW excitation source pairs is 50 pairs.

These electrodes are made of an aluminum thin film having a film thickness of 6000 mm formed by a vapor deposition method on a lithium niobate single crystal substrate (not shown) that transmits a 128-degree rotated Y-axis cut X-axis,
It is formed by photolithographic technology.

This embodiment is a 1F filter used in an intermediate frequency amplification stage of a color television receiver. The 1F filter is inserted between the tuner and the video and audio signal amplifying stage, and among the signals selected by the tube from the received signal, the video carrier signal, the chroma subcarrier signal, and the It is an object of the present invention to set an audio subcarrier signal to an appropriate relative level, suppress a signal of another channel that is not required, and transmit the signal to an amplifier at the next stage.

Usually, the level of the audio subcarrier is higher than the level of the video carrier in order to reduce the intermodulation of the video signal and the audio signal in the intermediate frequency amplification stage and to obtain appropriate audio sensitivity.
It is supposed to suppress 10 to 20 dB. Also, the amplitude frequency characteristic of the surface acoustic wave device is assumed to be asymmetrical from the relationship with the trap characteristic provided for the purpose of suppressing the video and audio carriers of the adjacent channels.

On the other hand, the surface acoustic wave device is composed of at least two electrodes, and in terms of ease of design and chip size, a regular electrode is used for one electrode as in this embodiment. Since the frequency characteristics of the normal type electrode are symmetric in amplitude, as described above, when an asymmetrical amplitude characteristic is required for the filter, the other electrode needs to have an asymmetrical amplitude characteristic.

The repetition period of the surface acoustic wave excitation source electrode having an asymmetric amplitude characteristic is not constant, therefore, it can not be an electrode finger width and space and lambda _0/8 constant.

In the present embodiment, the MEL is sufficiently suppressed in the vicinity of the specific frequency by setting the repetition period of the electrode finger corresponding to the specific frequency within the band of the device.

It was mentioned earlier that the IF filter transmits video, chroma and audio signals. Among them, the linearity of the phase characteristic is not so important in the audio signal band, but rather, the linear phase characteristic is required in the video signal band and the chroma signal band. In view of this point, the present embodiment is configured to sufficiently suppress the MEL in a band from the low frequency side (54.57 MHz) of the chroma subcarrier band to the high frequency side (59.25 MHz) of the video carrier band.

Since the interval between the surface acoustic wave excitation sources is not constant, the metallization ratio is not constant. In the above configuration, the MEL is canceled by the two electrode fingers at a frequency at which the surface acoustic wave wavelength is four times the electrode repetition period. Also, a sufficient degree of suppression can be obtained near the frequency.

As shown in FIG. 2, a sufficient MEL suppression effect of 41 dB or more (main signal ratio) was obtained in the frequency band from the chroma subcarrier to the video carrier.

 In addition, the worst value has a MEL suppression degree of 35 dB.

FIG. 3 shows the frequency characteristics of the surface acoustic wave device according to the present embodiment. As shown in Fig. 3, the ripple on amplitude characteristic 3 is 0.2d
Good characteristics were obtained in which the ripple on the group delay characteristic 4 was less than or equal to Bp-p and less than or equal to 60 nsp-p. Note that the insertion power loss of this device is 18.2 dB.

In the present embodiment, the repetition of electrode fingers of the same polarity is fixed, but it is clear that the repetition of electrode fingers of different polarity may be fixed.

Next, with reference to FIGS. 4 and 5, a description will be given of an improvement over the related art.

Conventionally, the electrode structure of the weighted electrode having the asymmetric amplitude characteristic has been designed according to the following method.

That is, 1/4 of the interval between adjacent excitation sources in the surface acoustic wave excitation source distribution is used as the electrode finger width and space, and therefore, the metallization ratio is constant at 0.5. FIG. 4 is a diagram showing the MEL frequency characteristic 2 of this electrode, -22 dB within the band.
The results are as follows, and it can be seen that the present embodiment has an improvement effect of 13 dB. FIG. 5 shows frequency characteristics of a surface acoustic wave device using a conventional electrode. In this case, the other configuration is the same as that of the first embodiment of the present invention. At this time, the ripple on the amplitude characteristic 3 was as large as 0.5 dBp-p, and the ripple on the group delay characteristic 4 was as large as 100 nsp-p.

Next, a second embodiment of the present invention will be described with reference to FIGS.

In this embodiment, the loss is improved by using a group-type unidirectional electrode as the interdigital electrode. FIG. 6 shows a schematic plan view of the weighting electrode 1, and FIG. 7 shows a frequency characteristic 2 of the MEL of the embodiment.

When a group-type unidirectional electrode is applied to a normal-type electrode (not shown), it generally has an asymmetrical characteristic in which the amplitude on the high-frequency side is large due to use in matching. Therefore, the weighting electrode used in combination with this requires higher asymmetry characteristics than in the first embodiment.

The group-type unidirectional electrode 1 includes a transmission electrode 5, a reflection electrode 6, and a meander electrode 7, and has a geometric phase difference determined by a center distance L between the transmission electrode 5 and the reflection electrode 6, and a transmission electrode 5 and a reflection electrode. The electric phase difference generated by a phase shifter (not shown) connected to the electrode 6 is appropriately set so that the surface acoustic wave propagates in only one direction. In this case, the loss can be minimized by matching the devices, and the unnecessary reflected wave (RW) determined by the matching condition with the load can be suppressed.

In this embodiment, the number of pairs of surface acoustic wave excitation sources per group is two. FIG. 6 shows only three groups as a part of the electrodes. Further, in order to suppress MEL, the metallization ratio is different for each electrode. At this time, good characteristics were obtained in which the degree of MEL suppression was 36 dB or less, the loss of the device was 6.3 dB, the amplitude ripple was 0.1 dBp-p or less, and the group delay ripple was 40 nsp-p or less.

As described above, according to the present invention, even in a surface acoustic wave device having an asymmetrical amplitude characteristic, it is possible to sufficiently suppress MEL and obtain a low ripple frequency characteristic. Here, the case of the intersection width weighting electrode has been described, but it is apparent that the present invention can be applied to a so-called phase weighting electrode in which the intersection width is constant and the repetition period of the surface acoustic wave excitation source is not constant.

〔The invention's effect〕

As described above, according to the present invention, MEL can be sufficiently suppressed even with an interdigital electrode having an asymmetrical amplitude frequency characteristic, so that a ripple in the frequency characteristic can be suppressed, and the effect of improving the performance of the filter is improved. is there.

[Brief description of the drawings]

FIG. 1 is a schematic plan view of a weighting electrode according to a first embodiment of the present invention, FIG. 2 is a MEL frequency characteristic diagram of the first embodiment, FIG. 3 is an amplitude frequency characteristic diagram of the first embodiment, and FIG. FIG. 5 is an MEL frequency characteristic diagram of the weighting electrode, FIG. 5 is an amplitude frequency characteristic diagram of the electrode, FIG. 6 is a schematic plan view of the weighting electrode of the second embodiment of the present invention, and FIG. FIG. 1 ... interdigital electrode, 2 ... MEL frequency characteristic, 3 ... amplitude frequency characteristic, 4 ... group delay frequency characteristic, 5 ... transmission electrode, 6 ... reflective electrode, 7 ... meander electrode.

Continuation of the front page (72) Inventor Jun Yamada 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside the Home Appliance Research Laboratory, Hitachi, Ltd. (56) References JP-A-52-15246 (JP, A)

Claims (1)

(57) [Claims] 1. A so-called so-called electrode having a plurality of interdigital electrodes on a piezoelectric substrate as input electrodes and output electrodes, wherein the interdigital electrodes are a set of two electrode fingers and alternately inverted in polarity. In a surface acoustic wave device which is a split-connect type electrode and at least one of the interdigital electrodes is an electrode having a non-uniform pitch portion in which the surface acoustic wave excitation source intervals are not equal, the non-uniform pitch portion is In the electrode having
The electrode finger repetition cycle of the electrode finger width of the two electrode fingers is
A surface acoustic wave device, wherein the surface acoustic wave device is constant in the electrode having the unequal pitch portion.